The invention relates to a method of making the cable core of a telecommunication cable water-tight in the longitudinal direction, in which the cable core consisting of stranded single wires is passed through a filling head, a filling material mainly consisting of hydrocarbons is supplied under pressure and in an excess quantity to the filling head at a temperature above the pour point of the material, is spread over the circumference of the cable core and is introduced into the cable core and the excess of filling material not absorbed by the cable core is drained away.
Telecommunication cables, which are generally buried in earth, must be protected as much as possible from the permeation of moisture and water into the cable and more particularly from the further penetration of water in the longitudinal direction of the cable. In cables whose single wires are provided with a paper insulation the paper also serves as a barrier against the penetration of water because the paper sheaths of the separate single wires will swell due to wetting and, apart from the moisture absorbed by the paper, form a practically adequate seal against a further penetration of water.
However, now the use of single wires with plastic insulation has become common practice, the problem of telecommunication cables being damaged by permeation of moisture and water has become very serious. Due to the fact that plastic material does not swell due to wetting, moisture and water, once permeated into the cable, can penetrate without hindrance along the single wires in the longitudinal direction of the cable. If such a penetration of moisture and water is not prevented, the electrical properties of the cable, such as capacitance and cross-talk, can deteriorate as a whole considerably. Furthermore, the water which has penetrated the cable can attack the single wires electrolytically through pin holes in the insulation and can lead to corrosion. Moreover, there is a risk of the water penetrating into the joint boxes, which may lead to short-circuits between the individual transmission circuits.
Various methods of making telecommunication cables longitudinally water-tight are known. According to one of these methods, a filling material having a base of petroleum jelly that may be mixed with polyethylene, is introduced into the cable core. This is effected at a temperature above the pour point of the filling material.
Such a filling material has a consistency such that a higher temperature of the order of 80.degree. C. it has a low dynamic viscosity of about 0.046 Pa.s and at a lower temperature of about 50.degree. C. it has a higher viscosity of about 9.15 Pa.s.
A method in which such a filling material with a petroleum jelly base is introduced into the cable core of a telecommunication cable is known from U.S. Pat. Nos. 3,789,099 and 3,876,487. In this known method, the heated filling material is supplied under pressure and in an excess quantity to the pressure filling chamber of a filling head, a pressure gradient being produced between the pressure filling chamber and a pressure relief chamber in order to obtain an axial flow of the filling material and to drain away the excess quantity of filling material supplied. This known method is based on a combination of pressure and speed of the filling material. Since the cable core in the pressure filling chamber is subjected to pressure on all sides, it is slightly pinched, as a result of which the penetration of filling material is impeded. In view of the pressure in the pressure filling chamber, this chamber has to be sealed, which gives rise to many problems. If the seals are seals which have a tight fit, there is a risk of the cable core being compressed and, in some cases, being damaged, which results in a poor filling of the cable core. If the seals are seals having an ample fit, there is a risk that insufficient pressure is built up in the pressure filling chamber to press the filling material into the cable core. This also leads to a poorly filled cable core. Moreover, the seals, which are of course adapted to the diameter of the cable core to be treated, must be replaceable in order that cable cores having different dimensions can be treated on the same apparatus.
British Patent Specification No. 1,502,375 discloses a method and an apparatus in which the last-mentioned disadvantage is obviated by the use of flexible expansble sleeves as seals. However, the further aforementioned disadvantages, i.e. pinching of the cable core, damage of the cable core and insufficient build-up of pressure in the pressure filling chamber, remain. The aforementioned problems arise to a greater extent during the step of filling multiwire cable cores, i.e. cable cores comprising a relatively large number of single wires.